The present invention is related to printing systems incorporating light emitting print bars as the imager, and more particularly, to a print system using LED print bars which are corrected for length changes and bowing of the image at a photosensitive image plane.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array and means are included for providing a relative movement between the linear array and the photoreceptor so as to produce a scanning movement of the linear array over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED array is advanced relative to the photoreceptor either continuously or in stepping motion. Each LED pixel in the linear array is used to expose a corresponding area in the photoreceptor to a value determined by image defining video data information.
In a color xerographic system, a plurality of print bars may be positioned adjacent the photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth print bar may be added if black images are to be created as well.
FIG. 1 shows a prior art single pass color configuration having three exposure stations, 10, 12, 14, each station including an LED array 10A, 12A, 14A. Each array is optically coupled to focus the array outputs on to the surface of a photoreceptor belt 16 forming three spaced latent images l.sub.1, l.sub.2, l.sub.3. The optical coupling is accomplished by a plurality of gradient index lens arrays 10B, 12B, 14B, the lens arrays sold under the name SELFOC.TM. a trademark of Nippon Sheet Glass Company, LTD. Upstream of each exposure station, a charge device 18, 20, 22 places a predetermined charge an the surface of belt 16. Downstream from each exposure station, a development system 26, 28, 30 develops a latent image of the last exposure without disturbing previously developed images.
With such a system as that disclosed in FIG. 1, each colored image must be precisely aligned such that all corresponding pixels in the image areas are registered. The LED array alignment requirements are that pixels of each array be aligned in the scan or Y-direction of FIG. 1 so that each active write length is equal. The array must also be aligned in the skew or X-direction. This alignment must be maintained through continuous revolutions (passes) of the photoreceptor.
There are several problems in the prior art when using multiple LED arrays writing on a photoreceptor in sequential imaging zones to produce an output print with multiple color. To maintain exact color registration of each image, typically to a tolerance of .+-.0.1.mu., the overall length of the write area, the pixel to pixel placement, and the straightness of the image line must all be within a required exacting tolerance. One of the most difficult manufacturing tolerances to achieve is the overall or active write length of the array. For example, for a 14.33" LED array with 300 spi resolution, 4300 pixels are aligned in the active write area and a .+-.15.mu. tolerance in write length is typical. A second problem is in maintaining the image line straightness. Imaged line deviation, known as bow, is a displacement perpendicular to the image line formed traversely to the photoreceptor surface, the bow occurring in the central portion of the imaged line.
According to the present invention, corrections to both of these print bar problems are accomplished by changing the physical properties of the gradient index lens arrays, which are optically coupled to the LED array outputs. It has been found that deforming the lens by applying a force at the lens array center or, alternately, at one or both ends of the array will either shorten or lengthen the active write length depending upon the direction that the force is applied and the magnitude of the force. It has further been found that applying a twisting force, or torque, to the center of the lens will move the central part of the image in a direction perpendicular to the image line, and thus, dependent upon the degree of torque, can reduce or eliminate a previously identified bow in the scan line.
More particularly, the present invention is directed towards a printer system including a line exposure apparatus for creating line images on a photoreceptor moving in a process direction comprising:
at least one image print bar including a linear array of a plurality of discrete light emitting sources, PA1 a linear lens array for focusing light from said emitting sources onto said photoreceptor, and PA1 means for deforming said lens array so as to alter the path of selected ones of said emitter sources being transmitted through said lens array, thereby altering the line image characteristics.
The following references have been identified in a prior art search.
U.S. Pat. No. 4,427,284 to Dannatt discloses an adjustment means for a fiber optic illuminator which includes a flexible lens assembly situated intermediate an array of fiber ends and a platen. A deflectable frame supports the flexible lens frame so that the flexible lens assembly can be transversely deflected to modify the linearity of the lens assembly without disturbing the focal adjustment thereof.
Japanese Patent No. 63-234522 to Hayashi discloses a reduction projection type exposure device including a spherically curved condenser lens 1 made of an elastically deformable transparent material. The lens has radially directed curvature changing arms 5 extending from its peripheral edges. By applying compressive or tensile loads to the arms, the curvature radius of the condenser lens can be controlled to equalize a pattern diameter over an exposure region.
Japanese Patent No. 60-217323 to Usui discloses an automatic focus optical device comprising a transparent elastic body 3 inside a cylindrical vessel 1 having a circular opening 2 in the top end thereof and a movable transparent plate 4 covering the bottom end. In accordance with the magnitude of a pressure applied to the movable plate, part of the elastic body projects from the opening in the top of the vessel in the shape of a convex lens or sinks in the shape of a concave lens.